This invention relates to polymers useful as the base resin in chemical amplification resist compositions suited for microfabrication. It also relates to chemical amplification resist compositions comprising the polymers, and a patterning process using the same.
In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. To the demand for a resist material with a higher resolution and sensitivity, acid-catalyzed chemical amplification positive working resist materials are effective as disclosed in U.S. Pat. Nos. 4,491,628 and 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography. Also, the change-over from i-line (365 nm) to shorter wavelength KrF laser (248 nm) brought about a significant innovation. Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.3 micron process, went through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule, with the trend toward a finer pattern rule being accelerated.
For ArF laser (193 nm), it is expected to enable miniaturization of the design rule to 0.13 xcexcm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198.
With respect to F2 excimer laser (157 nm) which is expected to enable further miniaturization to 0.10 xcexcm or less, more difficulty arises in insuring transparency because it was found that acrylic resins which are used as the base resin for ArF are not transmissive to light at all and those cycloolefin resins having carbonyl bonds have strong absorption. It was also found that poly(vinyl phenol) which is used as the base resin for KrF has a window for absorption in proximity to 160 nm, so the transmittance is somewhat improved, but far below the practical level.
An object of the invention is to provide a novel polymer having a high transmittance to vacuum ultraviolet radiation of up to 300 nm, especially F2 excimer laser beam (157 nm), Kr2 excimer laser beam (146 nm), KrAr excimer laser beam (134 nm) and Ar2 excimer laser beam (126 nm), and useful as the base resin in a chemical amplification resist composition. Another object is to provide a chemical amplification resist composition comprising the polymer, and a patterning process using the same.
It has been found that using as the base polymer an acrylate resin containing fluorinated alkyl groups in ester side chains, a resist material featuring high transparency is obtained.
In a first aspect, the invention provides a polymer comprising recurring units of the following general formula (1). 
Herein R1 to R3 are independently selected from the group consisting of hydrogen, fluorine, and straight, branched or cyclic alkyl and fluorinated alkyl groups of 1 to 20 carbon atoms, R4 is a monovalent fluorinated hydrocarbon group of 1 to 20 carbon atoms containing at least one fluorine atom, R5 is an acid labile group, R6 is an adhesive group, subscripts k, m and n are numbers satisfying 0 less than k+m+nxe2x89xa61. Preferably in formula (1), at least one of the units containing R4 to R6 has an alicyclic structure.
In a second aspect, the invention provides a resist composition comprising the polymer defined above.
Also provided is a chemically amplified, positive resist composition comprising (A) the polymer defined above, (B) an organic solvent, and (C) a photoacid generator. The resist composition may further include (D) a basic compound and/or (E) a dissolution inhibitor.
In a third aspect, the invention provides a process for forming a resist pattern comprising the steps of applying the resist composition onto a substrate to form a coating; heat treating the coating and then exposing it to high-energy radiation in a wavelength band of 110 to 180 nm or 1 to 30 nm through a photo mask; and optionally heat treating the exposed coating and developing it with a developer. Preferably, the high-energy radiation is an F2 excimer laser beam, Ar2 excimer laser beam or soft x-ray.
For improving the transmittance in proximity to 157 nm, reducing the number of carbonyl groups and/or carbon-to-carbon double bonds is contemplated to be one effective way. It was also found that introducing fluorine atoms into base polymers makes a great contribution to improved transmittance. In fact, poly(vinyl phenol) having fluorine introduced in its aromatic rings offers a transmittance nearly on a practically acceptable level. However, this base polymer was found to turn to be negative upon exposure to high-energy radiation as from an F2 excimer laser, interfering with its use as a practical resist. In contrast, those polymers obtained by introducing fluorine into acrylic resins or polymers containing in their backbone an alicyclic compound originating from a norbornene derivative have been found to be suppressed in absorption and overcome the negative turning problem. Especially when fluorine is introduced into ester side chains as in the present invention, the transmittance in proximity to 157 nm is significantly improved.